Advances in ceramic processing have permitted the replacement of various components of electrical and mechanical equipment with sintered ceramic parts. Ceramics have found widespread use in electronics as a substrate for integrated circuit packages. Ceramic substrates are typically formed by sintering slips produced from suspensions of ceramic particles. However, ceramics have exhibited problems of cracking, large and inconsistent shrinkage upon drying and firing, and nonuniform formability. Prior slip casting and tape casting systems have resulted in a green part with a low particle density, typically less than 55 percent of theoretical particle density. Low particle density results in low green strength, and the problem is exacerbated by the requirement of removing the binder used in the casting process.
Two types of dispersant mechanisms are known: steric dispersants, which utilize the size and conformation of the dispersant molecule; and electrostatic dispersants, which use electrical charge to cause dispersion. Typical steric dispersants include polymeric polyelectrolytes and carboxylic acids. Steric solvents are understood to operate by presenting (i) functional groups exhibiting strong surface interaction with the particle surface while being only marginally soluble in the solvent and (ii) stabilizing moieties that are highly soluble in the solvent. Electrostatic dispersants include acids and bases which operate to modify the pH of the suspending medium. Typically, dispersing agents have been used in aqueous and other solutions to create pourable suspensions of submicron particles having a maximum solids fraction of approximately 50 volume percent. Although this maximum solids fraction can be higher if the particles do not have a narrow size range, uniform size particles are required for uniform ceramics having uniform properties, such as reproducible shrinkage for net shape forming, and for high performance applications.
A higher volume percent of solids would result in less shrinkage upon drying and sintering, superior green strength, and a reduction in the probability of a large defect or void in particle packing. In addition, faster processing results from the reduced volume fraction of liquid.
Accordingly, there exists a need for a system for maintaining a maximum solids content of narrow size distribution in suspension, thereby maximizing green strength and minimizing shrinkage, while still maintaining the components in a flowable or pourable state.